This invention relates to novel azethidine derivatives and a process for producing these compounds from azethidine-2-carboxylic acids. More particularly, it relates to azethidine-2-carboxylic acid N-carboxyamino acid anhydrides represented by the following formula (1): 
and a process for producing these compounds from azethidine-2-carboxylic acids represented by the following formula (4): 
The azethidine-2-carboxylic acid N-carboxyamino acid anhydrides represented by the formula (1) are useful compounds as intermediates of drugs and the like.
The azethidine-2-carboxylic acid N-carboxyamino acid anhydrides represented by the formula (1) are novel compounds and no prior art has been reported hitherto concerning any processes for producing these compounds. Azethidine derivatives having peptide bond, which have a thrombin inhibitory activity, are highly useful as drug intermediates (U.S. Pat. No. 5, 714,485 and WO98/50420). These drug intermediates can be produced starting with (S)-azethidine-2-carboxylic acid which can be prepared by a publicly known method. However, condensation of an azethidine-2-carboxylic acid with an amine can be hardly performed in a single step in general. Namely, it is needed that the amine moiety of the azethidine-2-carboxylic acid is protected with a carbamate protective group, etc. followed by the condensation with the use of a condensing agent such as 1,3-dicyclohexylcarbodiimide (DCCD) and deblocking of the N-protective group. Therefore, it has been required to develop a more convenient production process.
This invention provides intermediate compounds useful in producing drugs, in particular, azethidine derivatives having peptide bond and a process for producing the same. Under the above-described conditions, the inventors have conducted intensive studies. As a result, they have found out that the azethidine derivatives having peptide bond can be efficiently produced via azethidine-2-carboxylic acid N-carboxyamino acid anhydrides, which are novel compounds, without resort to the deblocking procedure. The invention has been completed based on this finding.
Accordingly, the invention provides optically active azethidine-2-carboxylic acid N-carboxyamino acid anhydrides and racemate thereof, which are useful in producing azethidine derivatives having peptide bond, and a process whereby these compounds can be efficiently and economically produced on an industrial scale.
The optically active azethidine-2-carboxylic acid N-carboxyamino acid anhydrides and racemate thereof provided by the invention are compounds represented by the following formula (1): 
The term xe2x80x9coptically active compoundxe2x80x9d as used herein means one of the (S)- and (R)-enantiomers alone of an azethidine-2-carboxylic acid N-carboxyamino acid anhydride or a mixture of these enantiomers containing one of them at a higher mixing ratio. On the other hand, the term xe2x80x9cracematexe2x80x9d as used herein means a mixture containing the (S)- and (R)-enantiomers in the same amounts.
Next, the process for producing an azethidine-2-carboxylic acid N-carboxyamino acid anhydride will be described. 
An azethidine-2-carboxylic acid to be used as the starting material in the invention can be produced by, for example, the process described in WO98/47867.
An azethidine-2-carboxylic acid N-carboxyamino acid anhydride can be easily produced by reacting an azethidine-2-carboxylic acid with phosgene, di-phosgene and/or tri-phosgene.
Phosgene is used in an amount of generally from 0.1 to 10 mole equivalents, preferably from 1 to 5 mole equivalents, as much as the azethidine-2-carboxylic acid. Di-phosgene is used in an amount of generally from 0.1 to 10 mole equivalents, preferably from 0.5 to 5 mole equivalents, as much as the azethidine-2-carboxylic acid. Tri-phosgene is used in an amount of generally from 0.1 to 10 mole equivalents, preferably from 0.3 to 5 mole equivalents, as much as the azethidine-2-carboxylic acid.
Although the reaction may be carried out without using any solvents, it is usually performed in an organic solvent. Examples of the solvent, in case of using, involve ether solvents such as diethyl ether, diisopropyl ether, tetrahydrofuran, 1,4-dioxane and tertiary-butyl ethyl ether; ester solvents such as ethyl acetate, methyl acetate, propyl acetate and methyl propionate; hydrocarbon solvents such as pentane, hexane, heptane, cyclohexane, petroleum ether, toluene, benzene and xylene; nitrile solvents such as acetonitrile and propionitrile; ketone solvents such as acetone and ethyl methyl ketone; amide solvents such as N,N-dimethylformamide and N,N-dimethylacetamide; sulfoxide solvents such as dimethyl sulfoxide; and halogenated hydrocarbon solvents such as methylene chloride, chloroform and carbon tetrachloride. Among all, ether solvents are preferable and tetrahydrofuran is most desirable. Either one of these solvents or a mixture of two or more thereof may be used. The solvent is usually employed in an amount of from 1 to 50% by weight, preferably from 5 to 30% by weight, based on the azethidine-2-carboxylic acid, though the invention is not restricted thereto.
The reaction temperature varies depending on the reactants and reaction solvent employed. Usually it ranges from xe2x88x9250 to 100xc2x0 C., preferably from xe2x88x9210 to 80xc2x0 C. and still preferably from 30 to 60xc2x0 C.
The reaction time may be arbitrarily selected within a scope of usually from 0.5 to 10 hours, preferably from 2 to 7 hours and still preferably from 3 to 5 hours.
The compound (1) thus produced and its enantiomers can be collected as a compound having an arbitrarily high purity by optionally subjecting to publicly known separation and purification procedures such as concentration, extraction, chromatography and recrystallization.
The azethidine-2-carboxylic acid N-carboxyamino acid anhydrides produced by the process according to the invention can be easily introduced into azethidine derivatives (azethidine-2-carboxylic acid amides) having peptide bond by reacting with an amine. For example, (S)-azethidine-2-carboxylic acid (4-aminocyclohexyl) methanamide can be obtained by adding a THF solution containing (S)-azethidine-2-carboxylic acid N-carboxyamino acid anhydride to a liquid mixture of a 1 M sodium hydroxide solution containing (4-aminocyclohexyl)methanamine and tetrahydrofuran (THF) under ice-cooling followed by stirring.